Sauerbraten is multiplayer/singleplayer FPS game freeware game project, and
soon an RPG game project.  Sauerbraten is an experimental engine based on
Cube, can be seen as "Next-Gen Cube", or "Cube 2".  Much like cube, the aim
of this engine is not to produce the most eyecandy possible, but rather allow
map/geometry editing to be done dynamically in-game, and make map editing a
lot of fun.

WWW: http://sauerbraten.org/